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Asked: 2026-05-25T00:45:52+00:00

I am wondering why undefined == undefined but NaN != NaN .

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I am wondering why undefined == undefined but NaN != NaN.

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  1. Editorial Team

    Because that’s how it is defined in both the Abstract Equality Comparison Algorithm, and the Strict Equality Comparison Algorithm.

    If either operand to == or === is NaN, it returns false.

    Abstract

    • If Type(x) is Number, then
      • If x is NaN, return false.
      • If y is NaN, return false.
      • If x is the same Number value as y, return true.
      • If x is +0 and y is −0, return true.
      • If x is −0 and y is +0, return true.
      • Return false.

    EDIT: The motivation for the unequal comparison as noted by @CMS is compliance with the IEEE 754 standard.

    From the Wikipedia link provided in the comment below:

    …The normal comparison operations however treat NaNs as unordered and compare −0 and +0 as equal. The totalOrder predicate will order these cases, and it also distinguishes between different representations of NaNs and between the same decimal floating point number encoded in different ways.

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